Post pics of it, I should be able to help.
But I'll try without the pics.
Do you have a set of caliers or micrometers? If so, you need to set those to your size you want to check. The install the dail gauge into the shaft(if thats the style you have, thats why pics can help). Then slightly tighten the top collar nut making the shaft tight, but still needs to be moved, around the dail gauge. Nows the hardest part(but take your time, its worth it) set the tips(the measuring end of the gauge) into the claipers or mics you have set to your size. Now very gently move the dail gauge up or dwon in the shaft. You'll see the needle move. I like to "set" my needle directly at the top(thats just what I like, no need to do that). Once you have the needled at a spot where you liek at, set the gauge to zero, Just rotate the housing of the gauge to zero. With the "zero" set you can see the needle move off of the zero, telling you what size you have. Just do the math from the size you wanted to check from.
Does that help? Like Randy siad, its easy to use and set up, but hard to type out.